On the Surface Nature of Bimetallic PdZn Particles Supported on a ZnO-CeO2 Nanocomposite for the Methanol Steam Reforming Reaction

摘要

CO adsorption-as a molecular probe-was studied by transmission IR spectroscopy on pre-reduced Pd and bimetallic PdZn nanoparticles. Palladium was supported (2 wt% Pd) on pure CeO2, ZnO and a ZnO-CeO2 composite (atomic ratio Zn:Ce = 1:2). The Pd 3d(5/2) binding energy shift, together with the formation of metallic zinc were consistent with the development of a PdZn alloy over the zinc-containing supports at increasing reduction temperature, as revealed by XPS. Following H-2 reduction at 623 K the bimetallic particles showed only linear CO adsorption (COL) at initial contact time (10 Torr CO, 298 K), giving rise to a convoluted IR band ascribed to different Pd sites, where it was assumed that the Pd-Pd distances were larger than for pure Pd crystallites, indicating the presence of a PdZn alloyed surface. However, for longer exposure time to CO and/or higher superimposed pressure, the appearance of bridge and hollow coordinated CO (COB and COH, respectively) on the Pd sites suggested the degradation of the PdZn surface alloy, most likely due to the segregation of Pd surface patches. The temperature-programmed, dynamic isobaric adsorption of CO (TPA-CO), under flowing CO(1%)/He on the catalysts pre-reduced at 623 K (that is, for similar conditions to those found in the methanol steam reforming-MSR-process) showed faster desorption of COL as compared to COB + COH species for supported Pd/CeO2, as expected. However, the TPA-CO results on Pd/ZnO-CeO2 were atypical: even under the superimposed, low CO partial pressure, and for a temperature range similar to those found at high methanol conversion in the MSR reaction, the PdZn bimetallic surface nature was recovered, which could be an explanation of the good selectivity to CO2 of Pd/ZnO-based catalysts and-in particular-of the catalytically stable Pd/ZnO-CeO2 materials.